Erebus science – dating lava flows

Erebus science – dating lava flows

The sampling that is being carried out this year at Erebus also includes work on lava flows around the volcano. Dave Parmelee is a Masters student at New Mexico Tech., and over the past month he has been travelling all around the Erebus caldera by snowmobile and foot to collect samples from lava flows.

Dave completed his undergraduate study at the University of Connecticut in 2006. He worked for a large environmental consulting company, doing fieldwork in and around Connecticut. Dave and his girlfriend, Danielle, have done a lot of climbing and hiking around the USA, as well as spending some time in Guatemala. In 2010 they went to Peru, where they worked as volunteer scientists at a national park.

Earlier this week, Dave let me accompany him on one of his sampling trips. It was one of the highlights of this field season for me, to get out to one of the more remote areas of the volcano and see what he does. We walked around the Side Crater and down to the southern flanks of Erebus, where Dave collected some lava samples for Helium dating.

Dave searching for good outcrop on the southern slopes of Erebus, with Mount Terror in the distance.

Previous studies of Erebus lavas by Chris Harpel have involved Argon dating (see the reference below and the previous post for more information), which gives a possible age range for the flows; Dave is collecting samples for Helium (3He) and Chlorine (36Cl) dating on the same flows. Argon dating at Erebus has high uncertainties, resulting in overlap of the possible ages of different lava flows, so it’s hard to tell which order the flows were emplaced in. In addition to this, excess argon found in Erebus rocks can cause ages to appear higher than they actually are. Excess argon can be cleaned out in the lab, but the results from Argon dating at Erebus are considered maximum possible ages.

The isotopes used in 3He and 36Cl dating are formed when an outcrop is exposed to cosmic rays – so by measuring these isotopes, it is possible to calculate the length of time for which the lava flows have been exposed to the surface. Snow cover around Erebus this year is greater than usual, but this is helpful for Dave’s sampling, as those locations currently exposed are likely to be the same ones that are usually exposed.

There are many challenges in sampling from these lava flows. Snow and ice cover change from year to year. It takes two metres of snow cover to completely shield a rock from cosmic rays, so Dave has to make a judgement on whether a flow is likely to have been exposed constantly. Not all of the flows marked in the published map are well exposed. Dave uses a combination of this map and aerial photographs to identify sample locations.

Near the summit cone, there are large volcanic bombs in addition to the lava flows, so Dave has to distinguish between the two before he can select a place to sample. The location must be relatively flat, to ensure that there is little shielding from cosmic rays and that the production of cosmogenic isotopes will have been at the highest rate possible. He has to avoid locations with steep slopes or topography that may have blocked the outcrop from exposure to cosmic rays – the more open the area is, the better. Samples have to come from the top few centimetres, also to ensure exposure.

Another requirement for 3He dating is the abundance of suitable crystals: He isotopes for dating are, at Erebus, mainly found in a mineral called pyroxene. This tends to occur as small crystals, often incorporated into larger crystals of Erebus anorthoclase. Helium 3 occurs naturally, before exposure to cosmic rays, so the helium inherited from eruption needs to be isolated from that which is produced cosmogenically. By contrast, 36Cl is only produced by cosmic rays.

Once he has found a suitably flat area on the flow (with visible pyroxenes if the samples are to be used for 3He dating) Dave can take notes and photos of the sample location. Things he has to consider include whether the rocks are still in their original location; usual extent of snow cover; the shielding from the horizon, which he measures by taking the inclination of the topography above the flat horizon at different points; and the extent to which the rocks have been eroded.

Taking GPS measurements on lavas on the southwestern side of the caldera. Photo: Drea Killingsworth

At this stage, Dave gets to work with a hammer and chisel. Although the rock can be difficult to break up, it also requires some precision to avoid losing any pyroxenes for 3He dating. A minimum of 300 milligrams of pyroxene are required, which may mean carrying back a couple of kilograms of rock. Chlorine sampling, on the other hand, requires a bulk sample of rock, so can be slightly quicker. It can be several hours work to drive and then walk out to a sample location, find a good site, and get enough sample.

Over the next few months, Dave will be isolating the isotopes that are of interest – the 36Cl at New Mexico Tech., and the 3He at Woods Hole Oceanographic Institute in Massachusetts. The amounts of Cl and He isotopes can then be measured. The production rate for the cosmogenic isotopes is calculated, using published equations, from measurements of the isotopes in the lab combined with the field notes, but the result is an ideal production rate – in reality there may have been shielding from topography and snow which is no longer present. As a consequence the age calculated from the ideal production rate may well be younger than the actual age. Combined with the argon dates, this gives a better constraint on the ages of the flows and we can start to understand how periodic the eruptive behaviour of the volcano has been.

Despite all his experience in the outdoors, spending a month on a volcano has been a new and exciting experience for Dave. He has learned to drive a snowmobile on technical terrain and travelled out to parts of the caldera that have not been visited in a long time.

Dave had driven out to sample the Upper Tramways flow when I took this photo from the crater rim.

Thanks to Dave for the information and a great afternoon out in the field!

For the paper on Argon dating of Erebus lava flows: Harpel, C.J., Kyle, P.R., Esser, R.P., McIntosh, W.C., Caldwell, D.A., 2004. 40Ar/ 39Ar dating of the eruptive history of Mount Erebus, Antarctica: summit flows, tephra, and caldera collapse. Bulletin of Volcanology, 66(8): 687-702.

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